Conventional air separation units (ASUs) for the production of nitrogen, oxygen, and argon using cryogenic distillation technology are well known. ASUs typically separate air into its primary component gases at very low or cryogenic temperatures using one or more distillation columns. It is essential that certain impurities such as water vapor, carbon dioxide, nitrogen oxides, and trace hydrocarbons be removed from the compressed air feed prior to cryogenic distillation to avoid freezing of the impurities in the cryogenic equipment and potentially causing explosion. Any freezing will require stopping the process to remove the detrimental solid mass of frozen gases which is costly and can damage equipment. Generally, the content of water vapor and carbon dioxide in the compressed air feed stream must be less than about 0.1 ppm and about 1.0 ppm, respectively in order to prevent freeze up of these gases in an ASU.
The air feed stream is therefore cleaned or purified to remove these impurities prior to distillation typically by an adsorption process employing two or more vessels filled with beds of one or more adsorbents which selectively adsorb the impurities. Once an adsorption bed is saturated with impurities, it needs to be regenerated by removing the impurities so the bed is ready for further use.
Current commercial methods for this pre-purification of air generally include either one of or a combination of a cyclic pressure swing adsorption or temperature swing adsorption process. Pressure swing adsorption (PSA) uses a change in pressure, including vacuum, to regenerate the adsorbent and temperature swing adsorption (TSA) uses a thermal driving force such as a heated purge gas to desorb the impurities. The TSA process usually requires much lower amount of purge flow compared to PSA and affords a longer cycle time, typically in the range of 4 to 10 hours. The PSA process requires a greater amount of purge flow and affords a much shorter cycle time in the order of minutes. Moreover, there is no requirement for regeneration heat energy in PSA as opposed to TSA. Hence, when there is sufficient waste nitrogen available in a cryogenic air separation plant, the PSA process is usually a preferred option for air prepurification due to its simplicity, lower capital cost, and lower operating cost.
One disadvantage of the PSA process is that the adsorbents do not always get completely regenerated at the completion of the purge step and hence their dynamic capacity, the ability to remove the desired components, is lowered compared to the adsorbents regenerated in TSA processes. As a result, the PSA process is typically run for short cycle times necessitating that the bed undergoes blowdown (vent) and repressurization at fairly frequent intervals. During the blowdown step, there is a noticeable loss of air trapped within the void spaces of the vessel and piping as well as the air adsorbed on or within the adsorbents. This air loss, referred to by various terms such as blowdown loss, vent loss, or bed switch loss, represents a significant waste as the air is not utilized towards air separation downstream of the prepurifier. More significantly, there is an operational cost disadvantage as the air lost during bed switches utilizes valuable compression power. Accordingly, there is an increasing need to reduce this power requirement and increase the operational efficiency of the PSA prepurification process.
One way to lower the power requirement of the PSA process is to reduce the blowdown or bed switch loss described previously. This can be accomplished by reducing the frequency of bed blowdown and repressurization, for example by extending the cycle time for which the bed is kept online prior to being switched to regeneration. However, since the conventional commercial adsorbents, including zeolite-alumina composites, afford only modest dynamic working capacities for removal of the common air contaminants described above, an increase in cycle time would require either reducing the feed flow significantly at a fixed bed size or require a drastic increase in the bed size at a fixed feed flow rate. However, it has been found that by modifying the adsorbents employed to provide increased working capacities the improvements required can be achieved.
The use of zeolites, aluminas and certain composite adsorbents comprising zeolites and aluminas in PSA prepurifiers is known. Examples of prior art alumina-zeolite composites are disclosed in U.S. Pat. Nos. 5,779,767, 6,027,548, 6,358,302, and 6,638,340. Examples of alumina-zeolite bead mixtures are disclosed in U.S. Pat. No. 6,027,548, and EP 0904825 A2. However, none of these teachings use a composite adsorbent containing 10% or more of a metal oxide having a heat capacity of at least 20 cal/mol-° K (83.7 J/(mol·K)) in the adsorption process.